1. Technical Field
This disclosure is generally related to solid-state lighting devices having an adjustable output intensity.
2. Description of the Related Art
Traditional lighting systems adjust an output intensity of light by increasing or decreasing a line voltage, or by chopping the line voltage at some phase angle, either “phase cut” or “reverse phase cut”. Phase cut dimmers typically use a semiconductor device which is switched off at the zero crossing of the AC line voltage, and switched on at some phase angle later in the cycle to effectively modulate the power into the lighting device. For example, a typical low-cost household dimmer uses a TRIAC or SCR device to switch the AC power off during the line cycle. Reverse phase cut dimmers use more complex circuitry to turn on the semiconductor device at the zero crossing of the AC line, and turn off at some phase angle later in the cycle. Reverse phase dimmers have the advantage of lower electrical noise and less electrical stress on the components.
The lighting systems typically employ a dimmer at a location where a user switches the lighting system ON and OFF. The dimmer and ON/OFF switch is usually spaced from the light(s), for example mounted on a wall, where the light(s) are mounted on a ceiling or overhead. The dimmer is manipulated to increase or decrease the line voltage or the phase angle of the AC input sine wave routed to all lights that are downstream of the dimmer. Thus, where there are two or more lights on a single circuit, the dimmer does not enable individualized intensity adjustment to each of the lights. This type of setup with multiple lights on a single circuit is common. For example, in a warehouse, stadium, theater, box store, or grocery store, overhead lighting is electrically divided into tracks that are switched ON and OFF with just one or a few switches. Thus, manipulating a dimmer will change the output intensity of an entire track or an entire floor of lighting.
Solid state light sources, for example Light Emitting Diodes (LEDs), are typically powered by power converters that convert the AC input power to a controlled current that is kept below the maximum current level for the particular type of LED used. Such power converters may have provisions for adjusting the light output of the LEDs either by adjusting the current through the LEDs or by Pulse Width Modulating (PWM) the current through the LEDs. Such light sources have the same restriction as other types of light sources in that all of the light sources on the power line are dimmed to the same level, which is not desirable in many locations. For example, it may be desirable to dim some light sources in areas where less light is required, and not dim other light sources connected to the same power line.